The present invention relates to a method and apparatus particularly, though not exclusively, for the electro-deposition of metal coatings on plain bearings.
Plain bearings such as are used as crankshaft bearings in internal combustion engines for example, are generally semi-cylindrical in form, a pair of bearings forming a cylinder around each shaft journal usually being used. Each individual semi-cylindrical or half bearing generally comprises two or more layers including: a strong backing material such as steel for example; a lining of a bearing alloy such as an aluminium or copper alloy for example; and, frequently a so-called overlay coating of a softer metal alloy based on alloys of lead, tin, cadmium or zinc for example on top of the bearing alloy lining to provide bearing properties such as conformability and dirt embeddability for example.
Presently, the most commonly used method of providing the overlay coating is by electroplating from solutions containing ions of the required metals in appropriate concentrations in conjunction with either an inert or a consumable anode of the appropriate alloy. In the prior art plating processes, pairs of half bearings are held in stacks in jigs which allow free access of plating solution to the bearings. The jigs then being immersed in open tanks of various chemicals appropriate to the plating process being carried out. A typical prior art plating process may comprise the steps of: initially immersing the jigged bearings in a tank of cleaning fluid; transferring the cleaned bearings in their jig to a water wash tank; immersing the washed bearings in an acid etch tank; transferring the etched bearings to a water wash tank; transferring the washed bearings to a nickel plating tank; transferring the nickel plated bearings to a water wash tank; transferring the washed bearings to a tank for the electro-deposition of an alloy overlay coating; and finally transferring the overlay coated bearings to a water wash tank. Generally the transfer stages between tanks are effected by overhead cranes or conveyors and result in the fluid from one tank draining out of the jig as it is being raised with consequent splashing and risk of contamination and injury to the people operating the process. It must be born in mind that many of the cleaning, etching and plating solutions used comprise extremely toxic and harmful chemicals which, in addition to the danger posed by direct contact with people, may also produce harmful fumes.
A further disadvantage of the prior art process is that much of the time from initially jigging the bearings to removing the plated bearings from the jig is taken up in the transferring from tank to tank and in allowing time for each solution to drain as much as possible to minimise cross contamination between tanks.
A further disadvantage of conventional tank plating systems is that although the plating solutions in the tank are generally stirred, the current density able to be employed without xe2x80x9cburningxe2x80x9d of the overlay deposit is relatively low leading to relatively long plating times.
GB-A-2 181 744 shows a method of plating an internal surface of a hollow object by providing an internal anode and filling the internal volume of the object with a plating solution. However, the plating solution is only changed if it becomes exhausted and no indication of process conditions are given.
An object of the present invention is to provide a method and apparatus for the electroplating particularly, though not exclusively, of plain bearings, the method and apparatus being less hazardous to people than existing methods and apparatus.
A further object of the present invention is to provide a method and apparatus for the electroplating of plain bearings which is more economic and faster than known methods and apparatus.
Other objects of the present invention will become apparent from the disclosure of the present invention hereinbelow.
According to a first aspect of the present invention, a method for electroplating of a metal coating of at least a part of the surface area of an article includes the steps of: placing the article in a vessel, the vessel being provided with means to allow access of fluid to an interior volume thereof in which volume said article is contained and, means to allow egress of said fluid; providing said vessel with anode and cathode means such as to enable said article to become cathodic with regard to an anode extending into said interior volume; providing means to cause at least two different fluids including at least one plating solution to be introduced into said vessel in sequence, flow through said interior volume and to exit therefrom for a sufficient time to allow a required thickness of said metal coating to be deposited, said at least one plating solution flowing continuously through said vessel during deposition of said metal.
In the case of overlay plating of half bearings for example, the method may typically comprise the steps of clamping the half bearings together in either a generally cylindrical or semi-cylindrical stack for example within the interior volume of the plating vessel; passing a cleaning fluid through the vessel and past at least the bores of the bearings; then pumping water through the vessel to remove the cleaning fluid; next acid to etch the surface on which the overlay is to be deposited; next water to remove residual acid and wash the bearings; next a plating solution to deposit a thin layer of a so-called interlayer such as nickel or copper onto the bearing surface to act as a diffusion barrier between the bearing lining alloy and the overlay; next water to remove the residual interlayer plating solution; next overlay plating solution to deposit the overlay alloy per se; and, finally water to flush away residual overlay plating solution.
It should be emphasised that the above plating sequence is merely exemplary and that more, different or fewer fluid passing steps may be employed depending upon the specific bearing alloy being plated, the overlay alloy composition and the resulting bearing structure which is desired.
In the case of cylindrical or semi-cylindrical articles such as bearings for example, the anode has an elongate form extending into the plating vessel and may be generally coaxial with the bearing bore axis or vessel axis. It has been found that the method of the present invention is inherently a factor of at least about two or three times faster than in the conventional plating process where the jigged bearings are moved from tank to tank due to the higher current densities which may be employed allowing deposition of a given amount of metal in less time. However, it has been found that the speed of the electroplating deposition step per se may be further significantly increased by providing means to enable the anode surface to move such as by rotating, oscillating or reciprocating motion for example.
In one embodiment of the method of the present invention, the anode is provided with a paddle wheel at a position adjacent the fluid inlet means such that the fluid flow itself causes the anode to rotate. The higher the fluid flow rate, the faster the rate of rotation of the anode.
In another embodiment of the method and apparatus according to the present invention, the anode is driven by means of an electric motor thus, allowing independent control of rate of anode surface movement and fluid flow rate.
The use of high plating fluid flow rates such as in the range from about 15 to 40 l/min together with the use of a moving anode surface has enabled higher current densities in the range from about 5 to about 100 A/dm2 for any given step in the plating process to be employed thus producing rapid plating and an overall increase in the speed of the plating process by a factor of four or five. In the prior art plating processes, current densities were limited to about 7 A/dm2 otherwise xe2x80x9cburningxe2x80x9d of the plated coating occurred as evidenced by black sooty deposits forming on the surface. It is envisaged that flow rates of up to about 160 l/min may eventually be employed giving consequent increases in plating current densities which may be employed and hence reductions in process times.
In addition to the advantage of increases in plating process speed noted above, another considerable advantage of the present invention is that of safety whereby people operating the process are largely separated from the chemicals which are being used since they are contained within the plating vessel and stored in remote closed holding tanks being transported to and from the holding tanks by being pumped through conduits.
According to a second aspect of the present invention, there is provided apparatus for the deposition of a metal coating by electroplating on at least a part of the surface area of an article, the apparatus comprising: a vessel, the vessel having means to allow access of fluid thereinto, said article being contained in said vessel and, means to allow egress of said fluid from said vessel; anode and cathode means such as to enable said article to become cathodic with regard to an anode extending into said vessel; fluid supply means to supply at least two different fluids including at least one plating solution to said vessel in a predetermined sequence; control means to control the flow and sequence of said at least two different fluids to said vessel; means to enable continuous flow of said plating solution during deposition of said metal coating; holding tank means for holding at least some of said different fluids; and, electrical supply and control means to enable said electroplating of said article.
The different fluids may be held in separate tanks from which the fluids may be supplied from and returned to by suitable pump means and conduits connecting the vessel and tanks. The fluids may be supplied to the vessel via suitable valve control means which enable flow to and from the vessel to be initiated and terminated in a predetermined sequence.
The vessel may preferably be closed to the ambient environment so as to obviate spillage of fluid.
The vessel may also have a compressed air supply connected thereto to blow out the bulk of any residual fluids so as to minimise contamination of flushing water.
The fluid holding tanks may be provided with suitable monitoring means to ensure that the required concentrations of chemicals for example are maintained within desired limits.
The anode may be either an inert anode from a metal such as stainless steel or Hastelloy (trade name) for example or may be a consumable anode of the alloy which is being deposited as the overlay coating in the case of bearings, e.g. lead-tin-copper alloy.
The speed of the electroplating process has been found to be greatly enhanced by making the anode move during the process. In one embodiment of the apparatus of the present invention the anode has been provided with a paddle wheel which is driven by the incoming fluid as noted above. However, the speed of rotation is necessarily controlled inter alia by the fluid flow rate and the speed of rotation may thus not be optimum. The anode may alternatively be driven by motor means such as electric or air motor, for example, having speed control means. The anode may alternatively be oscillated about its axis or reciprocated in a direction substantially parallel to its axis or a combination of such motions. It is believed that the increase in plating speed due to anode movement is due to the prevention or limitation of diffusion layers around the anode being formed and thus hindering the free passage of metal ions to the article being plated.
Similarly, the high electrolyte flow rates ensure that there is always a fresh supply of metal ions at the article surface and prevents stagnation at the article surface.
In a preferred embodiment of the present invention, the fluids may be introduced into the vessel in a direction generally parallel to the vessel axis, the vessel axis being preferably generally vertical. The combination of the high plating fluid flow rate and moving anode surface causes turbulence which prevents the formation of a stagnating boundary layer of plating fluid adjacent both the anode surface and the article surface being plated thus, enabling the use of much higher plating current densities than have hitherto been possible.
Preferably, the anode cross section may be such as to cause turbulence in the flowing plating solution during movement thereof. Cross sectional shapes such as rectangular or xe2x80x9cSxe2x80x9d-shaped for example may be employed.
A yet further advantage of the apparatus and method of the present invention is that the floor space occupied is greatly reduced and holding tanks can be sited closer together and in remote and/or elevated positions away from people thus again improving safety and manufacturing area utilisation.
In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: